The present invention concerns a method in a headbox of a paper machine and the approach pipe system thereof. In the headbox, pulp suspension is fed through an approach pipe system and through a pressure screen arrangement thereof, if such is provided, to a distribution header part, from which the pulp suspension flow is conducted through a turbulence generator section and further to a lip section having a lip aperture. A pulp suspension jet is discharged from the lip aperture onto a forming wire or into a throat between a pair of forming wires.
The present invention further concerns a headbox arrangement in a paper machine and the approach pipe system thereof, comprising a pulp suspension feed pump and an approach pipe system. This pipe system communicates with a system attenuating pressure variations in the pulp suspension. The arrangement further comprises a distribution header part, a turbulence generator section connected thereto and to which a lip section terminating in a lip aperture is connected. A pulp suspension jet can be supplied onto a forming wire or into a throat between a pair of forming wires through the lip aperture.
In so-called variety production paper machines, which are used to make paper brands varying within very wide limits, e.g. in base weight thereof, the commonly encountered problem is that the ratio between the maximum flow rate q.sub.max and minimum flow rate q.sub.min of the pulp suspension from the headbox is too low. Such variety production machines are, as a rule, narrow and slow and would need to have a rather high value of the abovementioned flow rate ratio.
In existing headboxes, a flow ratio q.sub.max /q.sub.min .apprxeq.2 is usually achieved. With perforated roll or air cushion headboxes, a slightly higher flow rate ratio is achieved in some instances. With certain hydraulic headboxes, even higher flow ratios than mentioned above are obtained, but in that case an exchangeable turbulence generator has to be used and the height of the pulp suspension flow passage must be changed in addition.
In procedures and designs known in the art, satisfactory flow conditions can be obtained, even at high flow ratios, in the region of the turbulence generator and the lip cone. However, in the regions of the approach pipe system, the pressure variation attenuator, and the distribution header, major flow velocity digressions cause serious disturbances, which has not been able to be satisfactorily handled in designs of the prior art. If the flow velocity in the approach pipe system, the attenuator, and distributor header markedly differs downwardly from the rated values, soiling problems are incurred in the flow system. On the other hand, if the flow velocity in such units substantially deviates upwardly from the rated velocity, then excessive turbulence results which impairs the formation and homogeneity of the web. Therefore, any substantial exceeding of the flow ratio q.sub.max /q.sub.min .apprxeq.2 has become questionable in practice.
So-called multiple-layer headboxes are known in the art, being used in particular in cardboard machines. Such headboxes comprise a plurality of parallel pulp flow passages, each of which is connected to an individual pulp system so that different pulp can be supplied through each headbox layer in accordance with the layered design being produced.